U.S. patent number 8,929,197 [Application Number 14/035,713] was granted by the patent office on 2015-01-06 for method and system for an ofdm joint training and frequency tracking system.
This patent grant is currently assigned to Broadcom Corporation. The grantee listed for this patent is Broadcom Corporation. Invention is credited to Mark Kent.
United States Patent |
8,929,197 |
Kent |
January 6, 2015 |
Method and system for an OFDM joint training and frequency tracking
system
Abstract
Aspects of a method and system for an OFDM joint timing and
frequency tracking system may include tracking carrier frequency
and symbol timing in an Orthogonal Frequency Division Multiplexing
(OFDM) signal based on at least a reference symbol set. A receiver
frequency and timing may be adjusted based on the tracked carrier
frequency and symbol timing. The carrier frequency may be tracked
by generating an output signal as a function of a frequency offset
.DELTA.f, and the symbol timing may be tracked by generating an
output signal as a function of a guard time .DELTA.t.sub.g. The
received OFDM signal may be fast Fourier transformed to generate
the reference symbol (RS) set. The receiver frequency and timing
may be adjusted coarsely prior to fine adjustment. The coarse
receiver frequency and the timing adjustment may be based on
processing a primary synchronization signal and a secondary
synchronization signal.
Inventors: |
Kent; Mark (Vista, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Broadcom Corporation |
Irvine |
CA |
US |
|
|
Assignee: |
Broadcom Corporation (Irvine,
CA)
|
Family
ID: |
41376413 |
Appl.
No.: |
14/035,713 |
Filed: |
September 24, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140023162 A1 |
Jan 23, 2014 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
12184353 |
Aug 1, 2008 |
8559296 |
|
|
|
Current U.S.
Class: |
370/208; 370/210;
370/503 |
Current CPC
Class: |
H04L
27/2675 (20130101); H04L 27/265 (20130101); H04L
27/2649 (20130101); H04L 27/2662 (20130101); H04L
27/2657 (20130101) |
Current International
Class: |
H04J
11/00 (20060101) |
Field of
Search: |
;370/203-204,206,208-210,252,330,336,503,516 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2 079 178 |
|
Jul 2009 |
|
EP |
|
20050039263 |
|
Apr 2005 |
|
KR |
|
WO-2008/053889 |
|
May 2008 |
|
WO |
|
Other References
Santella, G., "A Frequency and Symbol Synchronization System for
OFDM Signals: Architecture and Simulation Results," IEEE
Transactions on Vehicular Technology 49:254-275, Rome, Italy (Jan.
2000). cited by applicant .
Communication pursuant to Article 94(3) EPO dated May 8, 2013.
cited by applicant .
Tanno et al.; "Physical Channel Structures and Cell Search Method
for Scalable Bandwidth for OFDM Radio Access in Evolved UTRA
Downlink." IEICE Transactions on Communications, vol. E90B, No. 12,
Dec. 2007, XP001509847. cited by applicant .
European Search Report for European Patent Application No.
09009039.0-2415, dated Dec. 17, 2009. cited by applicant .
Office Action for related Taiwanese Patent Application No.
098125936, mailed Feb. 14, 2014; 8 pages. cited by
applicant.
|
Primary Examiner: Mew; Kevin
Attorney, Agent or Firm: Sterne, Kessler, Goldstein &
Fox P.L.L.C.
Claims
What is claimed is:
1. A method for processing communication signals, the method
comprising: tracking a carrier frequency and a symbol timing in an
Orthogonal Frequency Division Multiplexing (OFDM) signal based on a
reference symbol set; coarsely adjusting a receiver frequency and a
receiver timing based on the tracked carrier frequency and the
tracked symbol timing, wherein the coarse adjustment includes
generating a coarse receiver frequency and a coarse timing
adjustment based on a primary synchronization signal and a
secondary synchronization signal; and finely adjusting the receiver
frequency and the receiver timing based on the tracked carrier
frequency and the tracked symbol timing.
2. The method according to claim 1, wherein the coarse adjustment
is performed prior to the fine adjustment.
3. The method according to claim 1, wherein the reference symbol
set comprises: a plurality of time-frequency slots configured to
change according to a time-frequency shift and pseudo-noise
sequence that modulates the reference symbol set.
4. The method according to claim 1, further comprising: Fast
Fourier Transforming the OFDM signal to generate the reference
symbol set.
5. The method according to claim 1, further comprising: controlling
the coarse and fine adjustment of the receiver frequency via a
receiver frequency oscillator.
6. The method according to claim 1, further comprising: controlling
the coarse and fine adjustment of the symbol timing via a timing
generator.
7. The method according to claim 1, wherein the tracking of the
carrier frequency and symbol timing comprises: comparing an input
clock signal to frequency and timing information extracted from the
OFDM signal utilizing the reference symbol set to generate
frequency error information and timing error information.
8. The method according to claim 7, wherein the tracking of the
carrier frequency comprises: generating an output signal as a
function of a frequency offset, the output signal being generated
based on the frequency error information.
9. The method according to claim 7, wherein the tracking of the
symbol timing comprises: generating an output signal as a function
of a guard time, the output signal being generated based on the
timing error information.
10. The method according to claim 7, wherein the tracking of the
carrier frequency comprises: generating an output signal as a
function of a frequency offset, the output signal being generated
based on the frequency error information; and wherein the tracking
of the symbol timing comprises: generating an output signal as a
function of a guard time, the output signal being generated based
on the timing error information.
11. A system for processing communication signals, the system
comprising: a frequency and timing module configured to track a
carrier frequency and a symbol timing in an Orthogonal Frequency
Division Multiplexing (OFDM) signal based on a reference symbol
set; and a receiver configured to: coarsely adjust a frequency and
a timing of the receiver based on the tracked carrier frequency and
the tracked symbol timing, wherein the coarse adjustment includes
generating a coarse receiver frequency and a coarse timing
adjustment based on a primary synchronization signal and a
secondary synchronization signal; and finely adjust the frequency
and the timing of the receiver based on the tracked carrier
frequency and the tracked symbol timing.
12. The system according to claim 11, wherein the receiver is
configured to perform the coarse adjustment prior to the fine
adjustment.
13. The system according to claim 11, wherein the reference symbol
set comprises: a plurality of time-frequency slots configured to
change according to a time-frequency shift and pseudo-noise
sequence that modulates the reference symbol set.
14. The system according to claim 11, wherein the receiver
comprises: a Fast Fourier Transform module that is configured to
Fast Fourier Transform the OFDM signal to generate the reference
symbol set.
15. The system according to claim 11, wherein the receiver
comprises: a receiver frequency oscillator, wherein the receiver is
further configured to control the coarse and fine adjustment of the
receiver frequency utilizing the receiver frequency oscillator.
16. The system according to claim 11, wherein the receiver
comprises: a timing generator, wherein the receiver is farther
configured to control the coarse and fine adjustment of the symbol
timing utilizing the timing generator.
17. The system according to claim 11, wherein the frequency and
timing module is configured to compare an input clock signal to
frequency and timing information extracted from the OFDM signal
utilizing the reference symbol set to generate frequency error
information and timing error information to track the carrier
frequency and the symbol timing.
18. The system according to claim 17, wherein the frequency and
timing module is configured to generate an output signal based on
the frequency error information, the output signal being a function
of a frequency offset.
19. The system according to claim 17, wherein the frequency and
timing module is configured to generate an output signal based on
the timing error information, the output signal being a function of
a guard time.
20. A system for processing communication signals, the system
comprising: a frequency and timing module configured to track a
carrier frequency and a symbol timing in an Orthogonal Frequency
Division Multiplexing (OFDM) signal based on a reference symbol
set; and a receiver configured to: generate the reference symbol
set and provide the reference symbol set to the frequency and
timing module, wherein the reference symbol set includes a
plurality of time-frequency slots configured to change according to
a time-frequency shift and pseudo-noise sequence that modulates the
reference symbol set; coarsely adjust a frequency and a timing of
the receiver based on the tracked carrier frequency and the tracked
symbol timing; and finely adjust the frequency and the timing of
the receiver based on the tracked carrier frequency and the tracked
symbol timing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY
REFERENCE
This patent application claims the benefit of U.S. patent
application Ser. No. 12/184,353, filed Aug. 1, 2008, entitled
"Method and System for an OFDM Joint Timing and Frequency Tracking
System" which is incorporated herein by reference in its
entirety.
This patent application makes reference to U.S. application Ser.
No. 12/184,383 (now U.S. Pat. No. 8,174,958), filed on Aug. 1,
2008; and U.S. application Ser. No. 12/184,410 (now U.S. Pat. No.
8,223,891), filed on Aug. 1, 2008, each of which is incorporated
herein by reference in its entirety.
FIELD OF THE INVENTION
Certain embodiments of the invention relate to signal processing
for communication systems. More specifically, certain embodiments
of the invention relate to a method and system for an OFDM joint
timing and frequency tracking system.
BACKGROUND OF THE INVENTION
Mobile communications have changed the way people communicate and
mobile phones have been transformed from a luxury item to an
essential part of every day life. The use of mobile phones is today
dictated by social situations, rather than hampered by location or
technology. While voice connections fulfill the basic need to
communicate, and mobile voice connections continue to filter even
further into the fabric of every day life, the mobile Internet is
the next step in the mobile communication revolution. The mobile
Internet is poised to become a common source of everyday
information, and easy, versatile mobile access to this data will be
taken for granted.
Third generation (3G) cellular networks have been specifically
designed to fulfill these future demands of the mobile Internet. As
these services grow in popularity and usage, factors such as cost
efficient optimization of network capacity and quality of service
(QoS) will become even more essential to cellular operators than it
is today. These factors may be achieved with careful network
planning and operation, improvements in transmission methods, and
advances in receiver techniques. To this end, carriers need
technologies that will allow them to increase throughput and, in
turn, offer advanced QoS capabilities and speeds that rival those
delivered by cable modem and/or DSL service providers. Recently,
advances in multiple antenna technology and other physical layer
technologies have started to significantly increase available
communications data rates.
Further limitations and disadvantages of conventional and
traditional approaches will become apparent to one of skill in the
art, through comparison of such systems with some aspects of the
present invention as set forth in the remainder of the present
application with reference to the drawings.
BRIEF SUMMARY OF THE INVENTION
A method and/or system for an OFDM joint timing and frequency
tracking system, substantially as shown in and/or described in
connection with at least one of the figures, as set forth more
completely in the claims.
These and other advantages, aspects and novel features of the
present invention, as well as details of an illustrated embodiment
thereof, will be more fully understood from the following
description and drawings.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
FIG. 1A is a diagram illustrating exemplary cellular multipath
communication between a base station and a mobile computing
terminal, in connection with an embodiment of the invention.
FIG. 1B is a diagram illustrating an exemplary MIMO communication
system, in accordance with an embodiment of the invention.
FIG. 2 is a diagram illustrating an exemplary OFDM symbol stream,
in accordance with an embodiment of the invention.
FIG. 3 is a diagram of an exemplary OFDM frequency and timing
acquisition and tracking system, in accordance with an embodiment
of the invention.
FIG. 4 is a flow chart illustrating an exemplary frequency and
timing acquisition and tracking, in accordance with an embodiment
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Certain embodiments of the invention may be found in a method and
system for an OFDM joint timing and frequency tracking system.
Aspects of the method and system for an OFDM joint timing and
frequency tracking system may comprise tracking carrier frequency
and symbol timing in an Orthogonal Frequency Division Multiplexing
(OFDM) signal based on at least a reference symbol set. A receiver
frequency and timing may be adjusted based on the tracked carrier
frequency and symbol timing.
The carrier frequency may be tracked by generating an output signal
that is a function of a frequency offset .DELTA.f, and the symbol
timing may be tracked by generating an output signal that is a
function of a guard time .DELTA.t.sub.g. The received OFDM signal
may be fast Fourier transformed to generate the reference symbol
(RS) set. The receiver frequency and timing may be adjusted
coarsely prior to fine adjustment. The coarse receiver frequency
and the timing adjustment may be based on processing a primary
synchronization signal and a secondary synchronization signal. The
reference symbol set may comprise a plurality of time-frequency
slots, which may change according to a time-frequency shift and PN
sequence that may modulate the reference symbols. The PN generated
sequences may be determined by base station identifier. This base
station identifier may be determined by the primary synchronization
signal (PSS) and secondary synchronization signal (SSS). The OFDM
signal may conform to a Universal Mobile Telecommunications
Standards (UMTS) long-term evolution (LTE) signal. The adjustment
of the receiver frequency may be controlled via a receiver
frequency oscillator (TXCO), and the adjustment of the timing may
be controlled via a timing generator.
FIG. 1A is a diagram illustrating exemplary cellular multipath
communication between a base station and a mobile computing
terminal, in connection with an embodiment of the invention.
Referring to FIG. 1A, there is shown a building 140 such as a home
or office, a mobile terminal 142, a factory 124, a base station
126, a car 128, and communication paths 130, 132 and 134.
The base station 126 and the mobile terminal 142 may comprise
suitable logic, circuitry and/or code that may be enabled to
generate and process MIMO communication signals.
Wireless communications between the base station 126 and the mobile
terminal 142 may take place over a wireless channel. The wireless
channel may comprise a plurality of communication paths, for
example, the communication paths 130, 132 and 134. The wireless
channel may change dynamically as the mobile terminal 142 and/or
the car 128 moves. In some cases, the mobile terminal 142 may be in
line-of-sight (LOS) of the base station 126. In other instances,
there may not be a direct line-of-sight between the mobile terminal
142 and the base station 126 and the radio signals may travel as
reflected communication paths between the communicating entities,
as illustrated by the exemplary communication paths 130, 132 and
134. The radio signals may be reflected by man-made structures like
the building 140, the factory 124 or the car 128, or by natural
obstacles like hills. Such a system may be referred to as a
non-line-of-sight (NLOS) communications system.
Signals communicated by the communication system may comprise both
LOS and NLOS signal components. If a LOS signal component is
present, it may be much stronger than NLOS signal components. In
some communication systems, the NLOS signal components may create
interference and reduce the receiver performance. This may be
referred to as multipath interference. The communication paths 130,
132 and 134, for example, may arrive with different delays at the
mobile terminal 142. The communication paths 130, 132 and 134 may
also be differently attenuated. In the downlink, for example, the
received signal at the mobile terminal 142 may be the sum of
differently attenuated communication paths 130, 132 and/or 134 that
may not be synchronized and that may dynamically change. Such a
channel may be referred to as a fading multipath channel. A fading
multipath channel may introduce interference but it may also
introduce diversity and degrees of freedom into the wireless
channel. Communication systems with multiple antennas at the base
station and/or at the mobile terminal, for example MIMO systems,
may be particularly suited to exploit the characteristics of
wireless channels and may extract large performance gains from a
fading multipath channel that may result in significantly increased
performance with respect to a communication system with a single
antenna at the base station 126 and at the mobile terminal 142, in
particular for NLOS communication systems. Furthermore, Orthogonal
Frequency Division Multiplexing (OFDM) systems may be suitable for
wireless systems with multipath.
FIG. 1B is a diagram illustrating an exemplary MIMO communication
system, in accordance with an embodiment of the invention.
Referring to FIG. 1B, there is shown a MIMO transmitter 102 and a
MIMO receiver 104, and antennas 106, 108, 110, 112, 114 and 116.
The MIMO transmitter 102 may comprise a processor block 118, a
memory block 120, and a signal processing block 122. The MIMO
receiver 104 may comprise a processor block 124, a memory block
126, and a signal processing block 128. There is also shown a
wireless channel comprising communication paths h.sub.11, h.sub.12,
h.sub.22, h.sub.21, h.sub.2 NTX, h.sub.1 NTX, h.sub.NRX 1,
h.sub.NRX 2, h.sub.NRX NTX, where h.sub.mn may represent a channel
coefficient from transmit antenna n to receiver antenna m. There
may be N.sub.TX transmitter antennas and N.sub.RX receiver
antennas. There is also shown transmit symbols x.sub.1, x.sub.2 and
x.sub.NTX, and receive symbols y.sub.1, y.sub.2 and y.sub.NRX.
The MIMO transmitter 102 may comprise suitable logic, circuitry
and/or code that may be enabled to generate transmit symbols
x.sub.i i.epsilon.{1,2, . . . N.sub.TX} that may be transmitted by
the transmit antennas, of which the antennas 106, 108 and 110 may
be depicted in FIG. 1B. The processor block 118 may comprise
suitable logic, circuitry, and/or code that may be enabled to
process signals. The memory block 120 may comprise suitable logic,
circuitry, and/or code that may be enabled to store and/or retrieve
information for processing in the MIMO transmitter 102. The signal
processing block 122 may comprise suitable logic, circuitry and/or
code that may be enabled to process signals, for example in
accordance with one or more MIMO transmission protocols. The MIMO
receiver 104 may comprise suitable logic, circuitry and/or code
that may be enabled to process the receive symbols y.sub.i
i.epsilon.{1,2, . . . N.sub.RX} that may be received by the receive
antennas, of which the antennas 112, 114 and 116 may be shown in
FIG. 1B. The processor block 124 may comprise suitable logic,
circuitry, and/or code that may be enabled to process signals. The
memory block 126 may comprise suitable logic, circuitry, and/or
code that may be enabled to store and/or retrieve information for
processing in the MIMO receiver 104. The signal processing block
128 may comprise suitable logic, circuitry and/or code that may be
enabled to process signals, for example in accordance with one or
more MIMO protocols. An input-output relationship between the
transmitted and the received signal in a MIMO system may be
specified as: y=Hx+n where y=[y.sub.1,y.sub.2, . . .
y.sub.NRX].sup.T may be a column vector with N.sub.RX elements,
.sup.T may denote a vector transpose, H=[h.sub.ij]: i.epsilon.{1,2,
. . . N.sub.RX}; j.epsilon.{1,2, . . . N.sub.TX} may be a channel
matrix of dimensions N.sub.RX by N.sub.TX, x=[x.sub.1, x.sub.2, . .
. x.sub.NTX].sup.T is a column vector with N.sub.TX elements and n
is a column vector of noise samples with N.sub.RX elements.
The system diagram in FIG. 1B may illustrate an exemplary
multi-antenna system as it may be utilized in a Universal Mobile
Telecommunication System (UMTS) Long-Term Evolution (LTE) system.
Over each of the N.sub.TX transmit antennas, a symbol stream, for
example x.sub.1(t) over antenna 106, may be transmitted. A symbol
stream, for example x.sub.1(t) , may comprise one or more symbols,
wherein each symbol may be modulated onto a different sub-carrier.
OFDM systems may generally use a relatively large number of
subcarriers in parallel, for each symbol stream. For example, a
symbol stream x.sub.1(t) may comprise symbols on carriers f.sub.m:
m.epsilon.{1,2, . . . M}, and M may be a subset of the FFT size
that may be utilized at the receiver. For instance, with FFT sizes
of N , N>M and may create guard-tones that may allow utilization
of variable bandwidth when deployed., for example, 64, 128, or 512
sub-carriers, The M sub-carriers may comprise a symbol stream
x.sub.1(t) , for example, that may occupy a bandwidth of a few
kilohertz to a few megahertz. Common bandwidth may be between 1 MHz
and up to 100 MHz, for example. Thus, each symbol stream may
comprise one or more sub-carriers, and for each sub-carrier a
wireless channel may comprise multiple transmission paths. For
example, a wireless channel h.sub.12 from transmit antenna 108 to
receive antenna 112, as illustrated in the figure, may be
multi-dimensional. In particular, the wireless channel h.sub.12 may
comprise a temporal impulse response, comprising one or more
multipath components. The wireless channel h.sub.12 may also
comprise a different temporal impulse response for each sub-carrier
f.sub.m of the symbol stream, for example x.sub.2(t) The wireless
channels as illustrated in FIG. 1B depict a spatial dimension of
the wireless channel because the transmitted signal from each
transmit antenna may be received differently at each receiver
antenna. Thus, a channel impulse response may be measured and/or
estimated for each sub-carrier.
FIG. 2 is a diagram illustrating an exemplary OFDM symbol stream,
in accordance with an embodiment of the invention. Referring to
FIG. 2, there is shown time-frequency axes 210; a symbol 0
comprising a cyclic prefix CP(0) 202a, an Inverse Fast Fourier
Transform (IFFT) symbol less CP(0) (IFFT(0)) 202b, and a cyclic
prefix CP(0) 202c, at frequency f1; a symbol 1 comprising a cyclic
prefix CP(1) 204a, an IFFT symbol less CP(1) (IFFT(0)) 204b, and a
cyclic prefix CP(1) 204c, at frequency f1. The IFFT(0) 202b and the
CP(0) 202c may together form a complete IFFT symbol for time domain
symbol 0 at frequency f1. The CP(0) 202a may be substantially
similar to CP(0) 202c. Similarly, the IFFT(1) 204b and the CP(1)
204c may together form a complete IFFT symbol for time domain
symbol 1 at f1, and CP(1) 202a may be substantially similar to
CP(1) 202c. Similarly, there is shown a symbol 0 comprising a
cyclic prefix CP(0) 206a, an Inverse Fast Fourier Transform (IFFT)
symbol less CP(0) (IFFT(0)) 206b, and a cyclic prefix CP(0) 206c,
at frequency f2. There is also shown a symbol 1 comprising a cyclic
prefix CP(1) 208a, an IFFT symbol less CP(1) (IFFT(0)) 208b, and a
cyclic prefix CP(1) 208c, at frequency f2. There is also shown an
FFT input window 214 (dashed line), a guard time .DELTA.t.sub.g, a
frequency offset .DELTA.f, and a slot marker 212. An LTE slot
structure, for example, may comprise 3, 6, or 7 OFDM symbols per
slot (two of which may be illustrated in FIG. 2) in the time
domain.
To generate an Orthogonal Frequency Division Multiplexing (OFDM)
symbol, an output of an IFFT comprising of IFFT(0) 202b and CP(0)
202c may be used to generate CP(0) 202a from CP(0) 202c, and append
it to IFFT(0) 202b. The cyclic prefix CP(0) 202 may be utilized to
avoid inter-symbol interference at an OFDM receiver, in the
presence of multi-path propagation in the wireless channel.
At an OFDM receiver, for example MIMO receiver 104, a sampled input
signal may be processed for each received symbol, for example over
an FFT input window 214. In order to decode the received symbols,
it may be desirable that the FFT input window 214 may be located in
a time domain symbol time slot, for example in time domain symbol
0. In particular, it may be desirable that the FFT input window 214
may not extend into a neighboring symbol, to avoid inter-symbol
interference. Furthermore, it may be desirable that the FFT input
window 214 may not overlap multiple symbols in the frequency
domain. Thus, the slot marker may indicate the beginning of a slot,
for example time domain symbol slot 0, as illustrated in FIG. 2.
The slot marker 208 together with .DELTA.t.sub.g may define the
position of the FFT input window 214 within a symbol slot in the
time domain. Similarly, a frequency carrier, for example f1 or f2,
together with a frequency offset .DELTA.f may determine the
location of the FFT input window 214 in the frequency domain. In
most instances, to keep interference due to the multipath channel
as low as possible at the receiver, it may be desirable to keep
.DELTA.t.sub.g and .DELTA.f small.
Thus, it may be desirable to acquire frequency and timing
information, and maintain frequency and timing tracking as they may
drift, for example, because of changes in propagation due to
mobility. In some instances, this may be combined with other
frequency and timing acquisition and tracking processes. In many
instances, coarse frequency and time synchronization may be
achieved via the Primary Synchronization Signal (PSS) and the
Secondary Synchronization Signal (SSS). Fine frequency and time
tracking may be acquired by a frequency acquisition and tracking
system, which may exploit reference signals (RS) embedded in an
OFDM signal. Reference symbols may be known symbols that may be
transmitted according to a known pattern over the time, frequency
and spatial resources in an OFDM system. In other words, reference
symbols may be transmitted at known timing instances, on known OFDM
carriers over certain antennas. By decoding and processing PS
symbols, the receiver may determine correct timing and frequency
information, for example, through coherent demodulation. RS symbols
may be transmitted from each antenna in a multiple antenna OFDM
system.
In the Enhanced Universal Terrestrial Radio Access (EUTRA)
interface, RS symbols may be generated based on cell-specific
hopping pattern, and comprise pseudo-noise (PN) covered sequences
of Reference symbols. In accordance with an embodiment of the
invention, the RS tone spacing may be 6 carriers, per transmit
antenna, for example. In accordance with various embodiments of the
invention, the RS tone spacing may be 2, or 4 carriers, for
example. The RS sequence may not be known to the mobile terminal
(user equipment, UE) during initial acquisition, for example
through the synchronization signals. In some instances, after
acquiring the primary synchronization signal (PSS) and the
secondary synchronization signal (SSS), the UE may have obtained
the cell-specific hopping pattern for the RS symbols, and the PN
covering sequence. This information may be used to obtain coarse
frequency and timing information. In accordance with various
embodiments of the invention, the RS symbols may then be decoded in
one or more frequency and timing acquisition and tracking block to
provide fine frequency and time tracking.
FIG. 3 is a diagram of an exemplary OFDM frequency and timing
acquisition and tracking system, in accordance with an embodiment
of the invention. Referring to FIG. 3, there is shown a common
receiver part 342, and a frequency and timing part 340. The
frequency and timing part 340 may comprise an RS error block 302,
and an RS timing and frequency loop 304. There is also shown an RS
set input, a frequency error signal f.sub.k , a timing error signal
e.sub.k, an output signal txco_accum, and an output signal
to_accum. The common receiver part 342 may comprise a timing
generator 312, an RS extraction block 314, a channel estimation
block 316, a receiver operations block (RXCVR) 318, a fast Fourier
transform (FFT) block 320, a buffering block 330, a sampling
bandwidth (BW) filter 332, an analog-to-digital block 334, a master
timer 336. and a TCXO 338. There is also shown an RF filter input,
a master timer output, a slot timing input from PSS, an RS set
output, a txco_accum signal, a to--accum signal, an rs_strb signal,
and a slot_strb signal.
The common receiver part 342 may comprise a timing generator 312,
an RS extraction module or circuit 314, a channel estimation block
316, a receiver operations block (RXCVR) 318, a fast Fourier
transform (FFT) block 320, a buffering block 330, a sampling
bandwidth (BW) filter 332, an analog-to-digital block 334, a master
timer 336, and a Temperature-Controlled crystal Oscillator (TCXO)
338. There is also shown an RF filter input, a master timer output,
a slot timing input from PSS, an RS set output, a txco_accum
signal, an rs_strb signal, and a slot_strb signal.
The frequency and timing part 340 may comprise suitable logic,
circuitry and/or code that may be enabled to extract frequency and
timing information from a received OFDM signal by processing an RS
set of signals, which may generate an output txco_accum that may
control the TCXO 338, for example. In addition, frequency and
timing part 340 may be enabled to generate an output signal
to_accum, which may be used to control system timing via the timing
generator 312, for example. The RS error block 302 may comprise
suitable logic, circuitry and/or code that may be enabled to track
frequency and timing offsets, for example .DELTA.f and
.DELTA.t.sub.g as illustrated in FIG. 2.
The common receiver part 342 may comprise suitable logic, circuitry
and/or code that may be enabled to receive radio frequency signals,
and process these signals. Processing may comprise FFT computation,
RS symbol extraction, channel estimation and other receiver signal
processing. The timing generator 312 may comprise suitable logic,
circuitry and/or code that may be enabled to generate timing
signals for RS extraction, rs_strb, and slot timing, slot_strb. The
signal slot_strb may be used to control FFT timing and frequency in
the buffering block 330, for example. The module or circuit 314 may
comprise suitable logic, circuitry and/or code that may be enabled
to extract the RS symbols from the FFT module or circuit 320
output.
The channel estimation module or circuit 316 may comprise suitable
logic, circuitry and/or code that may be enabled to estimate the
wireless channel response for RS symbols, which may be desirable
for receiver operations. The receiver operations module or circuit
(RXCVR) 318 may comprise suitable logic, circuitry and/or code that
may be enabled to measure and/or verify performance during receiver
operations. The fast Fourier-transform (FFT) module or circuit 320
may comprise suitable logic, circuitry and/or code that may be
enabled to generate a Fast Fourier Transform for an input signal.
The buffering module or circuit 330 may comprise suitable logic,
circuitry and/or code that may be enabled to interface with, for
example, the FFT engine. The buffering module or circuit 330 may
assist in dedicated processes, measurement processes, multimedia
broadcast multicast services (MBMS), and/or SSS processing for
hopping pattern determination. In some instances, each of the
processes may be performed in parallel.
The sample BW filter 332 may comprise suitable logic, circuitry
and/or code that may be enabled to filter the signal at its input,
and generate an output signal with limited bandwidth. The
analog-to-digital (A2D) module or circuit 334 may comprise suitable
logic, circuitry and/or code that may be enabled to receive an
analog RF-filtered signal and convert it to a digital signal
representation at the output, with an arbitrary number of bits. The
master timer 336 may comprise suitable logic, circuitry and/or code
that may be enabled to provide basic timing and/or frequency
functionality in the receiver. In some instances, the master timer
336 may count over 10 ms periods, and may be clocked at 30.72 MHz,
for example. The master counter may comprise a slot counter, and a
sample counter. The TXCO 338 may comprise suitable logic, circuitry
and/or code that may be enabled to generate a variable frequency
output signal, as a function of an input signal, for example a
voltage.
The common receiver part 342 may receive radio frequency signals,
and process these signals. Processing may comprise FFT computation,
RS symbol extraction, channel estimation and other receiver signal
processing. Some frequency and/or timing aspects of the common
receiver part 342 may be controlled by the frequency and timing
part 340. For example, the receiver subcarrier/carrier frequency,
for example f1 and/or f2 as illustrated in FIG. 2, may be
determined via the TXCO 338. Similarly, timing may be controlled
via the timing generator 312 via the signal to_accum.
The RS error block 302 may compare the frequency and timing of the
RS set input signal with, for example, an input clock signal and
may generate a frequency error signal f.sub.k, and a timing error
signal e.sub.k. The RS error block 302 may receive at its input a
set of RS symbols, which may be extracted in the RS extraction
block 314. The outputs of the RS error block 302 may be
communicatively coupled to an RS timing and frequency loop 304.
The RS timing and frequency loop 304 may track frequency and timing
offsets, for example .DELTA.f and .DELTA.t.sub.g as illustrated in
FIG. 2. The RS timing and frequency loop 304 may be enabled to
generate a timing output signal to_accum that may be a function of
.DELTA.t.sub.g, and a frequency output signal tcxo_accum that may
be a function of .DELTA.f. In accordance with an embodiment of the
invention, the txco_accum signal, may increase at a rate that is a
function of .DELTA.f, and may thus allow information about .DELTA.f
to be communicated to, for example, the TXCO 338, which in turn may
control the FFT input window's position in the frequency domain.
Similarly, the to_accum signal, may increase at a rate that is a
function of .DELTA.t.sub.g, and may thus allow information about
.DELTA.t.sub.g to be communicated to, for example, the timing
generator 312, which in turn may control the FFT input window's
position in the time domain.
The analog-to-digital (A2D) module or circuit 334 may receive an
analog RF-filtered signal and convert it to a digital signal
representation at the output, with an arbitrary number of bits. The
A2D 334 output may be communicatively coupled to an input of the
sample BW filter 332. The sample BW filter 332 may filter the
signal at its input, and generate an output signal with limited
bandwidth and/or attenuate certain frequency bands. The output of
the sample BW filter 332 may be communicatively coupled to a first
input of the buffering module or circuit 330. A second input to the
buffering module or circuit 330 may be communicatively coupled to
the output signal slot_strb from the timing generator 312. The
buffering module or circuit 330 may interface with, for example,
the FFT engine. The buffering module or circuit 330 may assist in
dedicated processes, measurement processes, multimedia broadcast
multicast services (MBMS), and/or SSS processing for RS PN sequence
determination. In some instances, each of the processes may be
performed in parallel. The output of the buffering module or
circuit 330 may be communicatively coupled to the FFT module or
circuit 320.
The FFT module or circuit 320 may generate a Fast Fourier Transform
for an input signal communicatively coupled from the buffering
module or circuit 330. Similar to the buffering module or circuit
330, the FFT module or circuit 320 may assist in signal processing
for dedicated processes, measurement processes, multimedia
broadcast multicast services (MBMS), and/or SSS processing for
radio time framing and RS PN sequence determination. A first output
of the FFT module or circuit 320 may be communicatively coupled to
a first input of the RS extraction module or circuit 314. The RS
extraction module or circuit 314 may extract the RS symbols from
the FFT module or circuit 320 output. In some instances, it may be
desirable to use a generated hopping sequence from the demodulated
base station signal and/or pseudo-noise (PN) covering for RS
decoding. The RS symbols extracted and output at the RS extraction
module or circuit 314 may be communicatively coupled to the input
of the frequency and timing part 340, and a channel estimation
module or circuit 316. The hopping pattern may be communicated to
the RS extraction module or circuit 314 via the rs_hopping_pattern
signal on a second input, as illustrated in FIG. 3. The RS
extraction module or circuit 314 timing may be controlled via a
third input signal rs_strb, communicatively coupled to an output of
the timing generator 312.
The timing generator 312 may generate timing and frequency signal
for RS extraction, rs_strb, and slot timing, slot_strb. The signal
slot_strb may be used to control FFT timing and frequency in the
buffering module or circuit 330. The timing generator 312 may
generate the output timing signals from a function of the master
timer input signal, slot timing (PSS), for timing and frequency
corrections and tracking. The master timer input signal may be
communicatively coupled to the master timer 336 output. The master
timer 336 may provide basic timing and frequency functionality in
the receiver. In some instances, the master timer 336 may count
over 10 ms periods, and may be clocked at 30.72 MHz, for example.
The master counter may comprise a slot counter, and a sample
counter. The input to the master timer 336 may be provided by an
operating RF crystal, the temperature-controlled crystal oscillator
(TXCO) 338, for example. The TXCO 338 may be communicatively
coupled to the threshold module or circuit 310 via the txco_accum
signal.
The channel estimation module or circuit 316 may estimate the
wireless channel response for RS symbols, which may be desirable
for receiver operations. The channel estimation output may be
communicatively coupled to the RXCVR 318. The RXCVR 318 may measure
and/or verify receiver performance functionality.
FIG. 4 is a flow chart illustrating an exemplary frequency and
timing acquisition and tracking, in accordance with an embodiment
of the invention. After initialization in step 402, the primary and
secondary synchronization signals, PSS and SSS respectively, may be
decoded in step 404. The decoding of the PSS and SSS may provide
coarse frequency and timing information for frame and slot
synchronization, for example. After step 404, the flow chart
transitions to step 406, where the FFT block 320, for example, may
utilize the frequency and time information to generate an FFT of a
received signal. The RS extraction block 314, for example, may
utilize the generated FFT to extract an RS set in the RS extraction
block 314. This RS set may be communicated to the frequency and
timing part 340. After step 406, the flow chart transitions to step
408.
In step 408, the frequency and timing part 340 may track the
frequency offset .DELTA.f and the timing offset .DELTA.t.sub.g
using the RS timing and frequency loop 304 as disclosed with
respect to FIG. 3, for example. Since the output signal txco_accum
may be generated as a function of .DELTA.f, the txco_accum output
signal from the threshold block 310 may carry information about
.DELTA.f to the TXCO 338. Similarly, the output signal to_accum may
be generated as a function of .DELTA.t.sub.g, and may thus carry
information about .DELTA.t.sub.g to the timing generator 312, for
example. Thus, the output signals txco_accum and to_accum may
enable tracking of the carrier frequency and timing, and may enable
adjustment of the receiver frequency and timing, for example in the
master timer 336 via the TCXO 338, and the timing generator 312. In
step 410, the master timer 336 may adjust the frequency based on
the input signal txco_accum, and the timing generator 312 may
adjust the FFT input window timing. After step 410, the flow chart
transitions to step 404. Alternatively, following step 410, the
flowchart transitions to step 412, where the flow chart ends.
In accordance with an embodiment of the invention, a method and
system for an OFDM joint timing and frequency tracking system may
comprise tracking carrier frequency and symbol timing in an
Orthogonal Frequency Division Multiplexing (OFDM) signal based on
at least a reference symbol set, as described for FIG. 2, for
example. A receiver frequency and timing may be adjusted based on
the tracked carrier frequency and symbol timing.
The carrier frequency may be tracked by generating an output signal
that is a function of a frequency offset .DELTA.f, and the symbol
timing may be tracked by generating an output signal that is a
function of a guard time .DELTA.t.sub.g. The received OFDM signal
may be fast Fourier transformed to generate the reference symbol
(RS) set, as described in FIG. 2 and FIG. 3. The receiver frequency
and timing may be adjusted coarsely prior to fine adjustment. The
coarse receiver frequency and the timing adjustment may be based on
processing a primary synchronization signal and a secondary
synchronization signal, as described for FIG. 4. The reference
symbol set may comprise a plurality of time-frequency slots, which
may change according to a hopping pattern that is determined by a
base station identifier. The OFDM signal may conform to a Universal
Mobile Telecommunications Standards (UMTS) long-term evolution
(LTE) signal. The adjustment of the receiver frequency may be
controlled via a receiver frequency oscillator (TXCO) 338, and the
adjustment of the timing may be controlled via a timing generator
312, for example.
Another embodiment of the invention may provide a machine-readable
and/or computer-readable storage and/or medium, having stored
thereon, a machine code and/or a computer program having at least
one code section executable by a machine and/or a computer, thereby
causing the machine and/or computer to perform the steps as
described herein for an OFDM joint timing and frequency tracking
system.
Accordingly, the present invention may be realized in hardware,
software, or a combination of hardware and software. The present
invention may be realized in a centralized fashion in at least one
computer system, or in a distributed fashion where different
elements are spread across several interconnected computer systems.
Any kind of computer system or other apparatus adapted for carrying
out the methods described herein is suited. A typical combination
of hardware and software may be a general-purpose computer system
with a computer program that, when being loaded and executed,
controls the computer system such that it carries out the methods
described herein.
The present invention may also be embedded in a computer program
product, which comprises all the features enabling the
implementation of the methods described herein, and which when
loaded in a computer system is able to carry out these methods.
Computer program in the present context means any expression, in
any language, code or notation, of a set of instructions intended
to cause a system having an information processing capability to
perform a particular function either directly or after either or
both of the following: a) conversion to another language, code or
notation; b) reproduction in a different material form.
While the present invention has been described with reference to
certain embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the present
invention. In addition, many modifications may be made to adapt a
particular situation or material to the teachings of the present
invention without departing from its scope. Therefore, it is
intended that the present invention not be limited to the
particular embodiment disclosed, but that the present invention
will include all embodiments falling within the scope of the
appended claims.
* * * * *